Synthesis of Aspirin using Salicylic acid is a good topic to write IB diploma chemistry Internal assessment.
You can synthesise Aspirin which is 2-Acetoxybenzoic acid through the acetylation of salicylic acid using acetic
anhydride or acetyl chloride as a result it is an esterification reaction. You can research on the topic and decide the suitable variables so you can frame a focussed research question.
The molecule of salicylic acid is actually 2-hydroxybenzoic acid. The following chemical reaction happens when salicylic acid and Ethanoyl chloride reacts.
CH3COCl + HOC6H4COOH = CH3COOC6H4COOH + HCl
You can read IB Diploma chemistry topics here.